Today, let's talk about window panes. The
University of Houston's College of Engineering
presents this series about the machines that make
our civilization run, and the people whose
ingenuity created them.

Glass has been made in
various forms for about 4500 years. But two
features of glass-making are suprising. One is that
it was slower and more difficult to develop than we
might realize. The other is that artisans could
make really fine glass tableware long before they
could make a decent windowpane.

The ancient Egyptians and Greeks made crude glass
decorations. But today's basic soda-lime glass --
made of sand, limestone, and sodium carbonate -- is
only about 2000 years old. The first glass of any
real quality was made by the Alexandrian Greeks in
North Africa around 300 BC. Soda-lime glass came
quickly on its heels, and both the Alexandrians and
the Romans after them made it into very fine
tableware. Fine tableware remained the main glass
product for a very long time.

Of course, the stained
glass art in Gothic cathedrals was highly
developed, and we might think that glass handling
had reached a high level of perfection by that
point. Actually, the thing that had reached a high
level of perfection -- and one that we may have
lost today -- was coloring the glass. A medieval
window admittted light, but it was seldom smooth
enough to provide a clear view. In fact, the
windows of cathedrals were a way to tell
beautifully lit bible stories to the faithful, who
generally couldn't read.

Medieval glassblowers made two kinds of flat glass
sheets. One was made by blowing a large cylinder --
then splitting it open and flattening it out while
it was still hot. The other kind -- called crown
glass -- was made by pouring molten glass out on a
turntable and letting centrifigal forces spread it
out from a central point. Crown glass became the
basic form of flat glass for a very long time. It
underwent considerable refinement; but, even as
late as 1800, most domestic windows still displayed
the characteristic umbilical imperfection -- called
a crown -- at their centers.

Actually, the French had developed the superior --
but pretty expensive -- plate glass process in the
latter 1700s. But mechanized methods for making
relatively inexpensive window glass weren't
developed until the early 1800s -- a scant 150 or
so years ago.

The lowly windowpane serves to remind us of how
much we take yesterday's great acts of inventive
genius for granted. A windowpane is the result of
an enormously complex set of chemical and
high-temperature mechanical-handling processes.

I'm John Lienhard, at the University of Houston,
where we're interested in the way inventive minds
work.

(Theme music)

You'll find one of the best accounts of the history
of glass and glass-making in the 1911
Encyclopaedia Britannica. For a revised
version of this episode, see Episode 1282.